1. Field of the Invention
This invention relates to x-ray projection imaging systems. In a primary application the invention relates to obtaining projection x-ray images of a specific material in a volume of the body. In another application the invention relates to obtaining isolated images of blood vessels without bone and soft tissue using non-invasive injections of iodinated contrast material.
2. Description of Prior Art
At present images of blood vessels are made using invasive procedures where a catheter is inserted into the vessel and large amounts of an iodinated contrast material is inserted. The large amount is necessary to sufficiently attenuate x-rays so that the vessels will be visible. There have been a number of efforts to provide greater sensitivity to the iodine so that the vessels can be imaged with a non-invasive injection of iodine into a peripheral vein. Thusfar these have been relatively unsuccessful. In general much of the efforts at obtaining improved sensitivity to iodinated contrast agents concentrated on utilizing the unique energy spectral characteristics of iodine compared to those of body tissues.
The earliest approaches used relatively slow techniques which required long exposure times. These long exposure times are inadequate for imaging vessels which exhibit significant motion, as many do. Some of these early approaches used low-power monoenergetic sources on either side of the K edge of iodine. A system of this type using mechanically scanned x-ray beams and mechanical analog computers is described in Vol. VI of the Advances in Biological and Medical Physics published by the Academic Press in the chapter by B. Jacobson and R. Stuart Mackay on "Radiological Contrast Enhancement Methods." The section labelled "Dichromography," from page 224 to 231 describes a system using a special x-ray source with two fluorescent secondary emitters which alternately generated two monochromatic x-ray beams having energies on either side of the iodine K absorption edge. At each point wedge shaped materials of known composition are translated across the beam until the output beam reaches its predetermined value. The thickness of the wedges is then a direct indication of the amounts of the particular material present. A similar approach is described by B. Jacobson in the American Journal of Roentgenology, Vol. 91, January 1964, entitled "X-ray Spectrophometry in Vivo." In this paper a similar secondary emissive source alternately produced three monoenergetic wavelengths. Wedges of a soft tissue equivalent, bone and iodine were used in a mechanical analog computer to determine the amount of these body materials at each point in the scan.
Although these systems gave interesting results they suffered from using low power sources which required a long time to create an image. Normal heart and respiratory motions during the scanning time resulted in a blurred, low-resolution image. The low power of the sources was due to the use of monoenergetic beams.
At present, sufficient x-ray power for rapid exposures can only be derived from x-ray tubes. There is an early brief reference to the use of x-ray tube sources in an energy selective system in Appendix II of "Television X-ray Movies: Dose and Contrast Factors" by R. S. Mackay in the IRE Transactions on Medical Electronics, Vol. ME-7, No. 2, Apr. 1960, pp. 80-86. In this paper a heterochromatic source is used to produce different x-ray wavelengths. The various transmitted energies are detected using a scintillation detector and pulse height analyzer. A scanning beam is used, thus analyzing the tissue one point at a time. Since pulse height analyzers must isolate single photon events, this system is very slow. The long scan time makes it impractical for radiographic studies.
A number of studies have been made of isolating the iodinated contrast material by filtering an x-ray tube source on either side of the K edge of iodine. One such system is described in U.S. Pat. No. 3,854,049 issued to Charles A. Mistretta. In this system the x-ray beam is alternately filtered by a rotating filter wheel. Each filter produces a relatively narrow spectrum on either side of the K absorption edge of iodine. The television fluoroscopic images resulting from each filter are subtracted to provide an isolated image of the iodine.
These systems have thusfar been unable to provide non-invasive images of small vessels such as the coronary arteries for a number of reasons. Imaging detectors such as television-fluoroscopic systems or screen-film systems contain significant amounts of additive noise over and above the fundamental quantum noise due to the captured x-ray photons. These additive noise components obscure the detection of small difference signals as would be encountered with small vessels. Also, heavily filtered quasi-monoenergetic beams are relatively low in power, thus reducing the desired signal intensity. In addition, x-ray energies in the vicinity of the iodine K edge of 33 kev have relatively low transmission through the body since the attenuation coefficient of tissue is relatively high in this region. This further reduces the intensity of the desired signals as compared to the noise.
In general, imaging detectors, which simultaneously acquire the entire x-ray image, have a number of serious problems in systems attempting to isolate small amounts of iodine. As previously indicated, they have additive noise problems. Also, these systems are subject to significant amounts of scattered radiation since the entire volume is being irradiated simultaneously. Each imaging point thus receives scattered radiation from the entire volume. This scatter is reduced by the scatter-reducing grids, but it remains a significant noise source. In addition, imaging detectors are faced with fundamental tradeoffs between resolution and capture efficiency. If thick screens are used the capture efficiency of photons will be high, but the increased spreading of the light results in poor resolution. Conversely, thin screens produce high resolution and low capture efficiency. It is for these reasons that the present invention utilizes non-imaging detectors where a subsection of the image is acquired at each time.
The use of filtered spectra on either side of the K edge results in some error because of the finite width of the energy spectrum. These errors are particularly large in the case of bone whose attenuation coefficient varies significantly in this region. These systems can be compensated for by the use of a third energy as described in a publication by F. Kelcz, C. A. Mistretta and S. J. Riederer, "Spectral Considerations for Absorption Edge Fluoroscopy" in Medical Physics, Vol. 4, pp. 26-35, 1977. The output from this third energy can compensate for the bone errors, but the other problems, such as those due to the use of imaging detectors, remain.
A generalized system providing isolated images of specific materials is described in U.S. Pat. No. 3,848,130 issued to Albert Macovski. In this system transmission measurements are made at various energies and processed to provide isolated material images. Methods are shown for processing signals obtained from broad energy spectra so that narrow band filtering is not required. However, this system uses imaging detectors, including television fluoroscopy and film, so that the previously described problems will prevent the non-invasive visualization of small vessels.
In general all of the systems in the prior art used relatively low energies for the isolation of iodine in an attempt to take advantage of the large changes in the iodine attenuation coefficient. However, these lower energies present significant problems as was previously mentioned. The relatively low transmission of the body reduces the detected signals and increases the quantum noise. Also, at these lower energies, the mass attenuation coefficient of bone and soft tissue are quite different. This difference makes it difficult to provide an isolated iodine image in the presence of both soft tissue and bone.
In general, in the diagnostic region of the energy spectrum, the attenuation coefficients of all materials can be decomposed into a photo-electric component and a Compton scattering component. In U.S. Pat. No. 4,029,963 issued to R. E. Alvarez and A. Macovski, measurements are made at two energies spectra with the resultant signals processed to produce images of the photoelectric and Compton-scattering components. This system is used for both single projection images and for cross-sectional images using computerized tomography. In the single projection images, however, imaging detectors are used with all of the attendant problems of limited performance previously described. This patent also describes the concept of energy-sensitive integrating detectors. A dual detector is shown which simultaneously provides separate measurements at high and low energy spectra.
To obtain the desired accuracy it is important to obtain efficient, low-noise x-ray measurements which are essentially free of scatter. These are presently obtained in all of the commercial systems of computerized tomography. The scatter is not a problem since a relatively small region is irradiated at any one time using either a pencil beam or a sheet beam. The attendant collimation at the detector all but eliminates the scatter. The detectors used are individual high-efficiency detectors arranged in arrays. The thickness of the detectors determines the efficiency and the lateral dimensions determines the resolution. These are relatively independent since the detectors are isolated and provide separate measurements. Most important, the detectors are essentially free of additive noise. The relatively large electrical signals produced by either photo-multipliers or Xenon detectors are limited solely by quantum noise so that the accuracy is determined solely by the amount of radiation.
These accurate detectors are beginning to be used for single projection radiography. In the Microdose system produced by American Science and Engineering and in the Scout View System produced by General Electric the computerized tomography instrument is modified to provide a projection image. The body is indexed through the fan beam with the detector array providing a sequence of line information to produce a transmission image. Initially these transmission images were designed to identify the anatomy so as to facilitate the choice of the desired level for the cross-sectional image. Because of the detection accuracy, however, they have provided high-quality transmission images which have diagnostic value in their own right. These images, however, are not capable of non-invasive visualization of small vessels because of the intervening tissue. These are single energy spectra systems which are not capable of obtaining isolated images of specific materials.